The present disclosure relates to a measuring system for dimensionally measuring an object. This measuring system is preferably realized in the form of a coordinate-measuring machine.
A multiplicity of measuring systems for dimensionally measuring an object are already known from the prior art. In dimensional metrology, a wide variety of measuring methods are used to measure objects of any type in terms of their geometry and the dimensions.
A method that is used frequently in the construction of vehicle bodies is for example the measuring method using stripe light projection, in which the three-dimensional geometry of the workpiece to be measured can be calculated on the basis of customary triangulation methods. Such methods are known for example from U.S. Pat. No. 7,414,732 B2 and U.S. Pat. No. 8,502,991 B2.
As an alternative, in particular for measuring applications requiring very high precision, coordinate-measuring machines are typically used. In such coordinate measuring machines, different kinds of sensors may be used to capture the coordinates of the object to be measured. For example, measuring sensors as are sold by the applicant under the product designation “VAST XT” or “VAST XXT” are known in this regard. Here, the surface of the workpiece to be measured is scanned with a stylus, the coordinates of which in the measurement space are known at all times. Such a stylus may also be moved along the surface of a workpiece, such that a multiplicity of measurement points may be captured at set time intervals during such a measuring process within the scope of what is known as a “scanning method”.
Furthermore, it is known practice to use optical sensors that facilitate contactless capturing of the coordinates of the measurement object. One example of such an optical sensor is the optical sensor sold by the applicant under the product designation “ViScan”.
Furthermore, a multiplicity of coordinate measuring machines exist which use both tactile and optical sensors. This kind of coordinate measuring machine is also known as a multi-sensor coordinate measuring machine.
Coordinate measuring machines generally involve complex engineering work in order to link what is known as the tool centre point of the sensor or sensors to the material measures in the machine during dimensional measurements. The machine requires these material measures during measurement of components to establish a spatial relationship between the measurement positions at which the individual probing operations take place as part of a measurement plan. The material measures are required as orientation aid, as it were, so that the machine knows the position and spatial orientation of the sensor and the object to be measured.
The material measures in coordinate measuring machines are generally in the form of linear material measures and/or rotary angle sensors, with respect to which the machine measures the displacement that is necessary to be able to bring the sensor or sensors to different locations relative to the object to be measured.
The mechanics and the material measures are designed regularly carefully and hence expensively. The underlying reason is that the respective material measures are inevitably situated not near the tool centre point, but typically at the edge of the measurement volume and, as viewed from the tool centre point, away from the mechanics for introducing the respectively required degree of freedom of location displacement. Were this chain configured cheaply, i.e. mechanically not absolutely exactly or even to be “rickety”, no reliable connection to the material measures and thus no exact measurement of the object would be possible. This problem generally affects all measurement machines having sensors with a measurement region that is smaller than the desired measurement volume or than the size of the object to be measured.
In a large number of coordinate measuring machines of a wide variety of structural types, movable object carriers are used for example, by way of which the object to be measured is displaced along one, two or three axes relative to the measuring sensor to perform the multiplicity of probing operations that are specified in the test plan. The displacement mechanics of these object carriers must then make possible extremely exact positioning of the object to be measured, wherein the respective position must be continuously traceable with a high degree of precision, because otherwise the position and spatial orientation of the object relative to the sensor to be measured is unknown. It is easily understandable that such displacement mechanics, as are also used elsewhere in coordinate measuring machines, for example for the position change of the measuring sensor, are highly complicated and thus expensive.
In contrast, however, there is a continued effort to be able to save costs in terms of production of such coordinate measuring machines. However, this must not be at the expense of the measurement accuracy.